1.一对一关系

2.一对多查询

3.多对多查询

4例题演示

前置准备

对应的依赖


    <dependencies>
        <!-- mysql驱动 -->
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>8.0.17</version>
        </dependency>
        <dependency>
            <groupId>org.mybatis</groupId>
            <artifactId>mybatis</artifactId>
            <version>3.4.6</version>
        </dependency>
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>4.12</version>
        </dependency>
        <dependency>
            <groupId>log4j</groupId>
            <artifactId>log4j</artifactId>
            <version>1.2.17</version>
        </dependency>
    </dependencies>

1.准备四张表

user

orders表与user 一对多一对一关系

sys_role表

sys_user_role表 user 和sys_role的中间表多对多关系

2.实体类

User(一对多，一个user可以有多个order)

package com.itheima.domain;

import java.util.Date;
import java.util.List;
public class User {

    private int id;
    private String username;
    private String password;
    private Date birthday;

    //描述的是当前用户存在哪些订单
    private List<Order> orderList;

    //描述的是当前用户具备哪些角色
    private List<Role> roleList;

    public List<Role> getRoleList() {
        return roleList;
    }

    public void setRoleList(List<Role> roleList) {
        this.roleList = roleList;
    }

    public List<Order> getOrderList() {
        return orderList;
    }

    public void setOrderList(List<Order> orderList) {
        this.orderList = orderList;
    }

    public Date getBirthday() {
        return birthday;
    }

    public void setBirthday(Date birthday) {
        this.birthday = birthday;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }

    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", username='" + username + '\'' +
                ", password='" + password + '\'' +
                ", birthday=" + birthday +
                ", roleList=" + roleList +
                '}';
    }
}

Order（一对一,一个order只有一个user）

package com.itheima.domain;

import java.util.Date;

public class Order {

    private int id;
    private Date ordertime;
    private double total;

    //当前订单属于哪一个用户
    private User user;

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public Date getOrdertime() {
        return ordertime;
    }

    public void setOrdertime(Date ordertime) {
        this.ordertime = ordertime;
    }

    public double getTotal() {
        return total;
    }

    public void setTotal(double total) {
        this.total = total;
    }

    public User getUser() {
        return user;
    }

    public void setUser(User user) {
        this.user = user;
    }

    @Override
    public String toString() {
        return "Order{" +
                "id=" + id +
                ", ordertime=" + ordertime +
                ", total=" + total +
                ", user=" + user +
                '}';
    }
}

Role（多对多）

package com.itheima.domain;

public class Role {

    private int id;
    private String roleName;
    private String roleDesc;

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getRoleName() {
        return roleName;
    }

    public void setRoleName(String roleName) {
        this.roleName = roleName;
    }

    public String getRoleDesc() {
        return roleDesc;
    }

    public void setRoleDesc(String roleDesc) {
        this.roleDesc = roleDesc;
    }

    @Override
    public String toString() {
        return "Role{" +
                "id=" + id +
                ", roleName='" + roleName + '\'' +
                ", roleDesc='" + roleDesc + '\'' +
                '}';
    }
}

3.对应接口

UserMapper（与order一对多、role多对多）

package com.itheima.mapper;

import com.itheima.domain.User;

import java.util.List;

public interface UserMapper {

    public List<User> findAll();

    public List<User> findUserAndRoleAll();

}

OrderMapper(与user 一对一关系)

package com.itheima.mapper;

import com.itheima.domain.Order;

import java.util.List;

public interface OrderMapper {

    //查询全部的方法
    public List<Order> findAll();

}

4.配置

抽取jdbc jdbc.properties

jdbc.driver=com.mysql.cj.jdbc.Driver
jdbc.url=jdbc:mysql://localhost:3306/test?useSSL=false&serverTimezone=Asia/Shanghai
jdbc.username=root
jdbc.password=root

日志 log4j.properties

### direct log messages to stdout ###
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.Target=System.err
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=%d{ABSOLUTE} %5p %c{1}:%L - %m%n

### direct messages to file mylog.log ###
log4j.appender.file=org.apache.log4j.FileAppender
log4j.appender.file.File=c:/mylog.log
log4j.appender.file.layout=org.apache.log4j.PatternLayout
log4j.appender.file.layout.ConversionPattern=%d{ABSOLUTE} %5p %c{1}:%L - %m%n

### set log levels - for more verbose logging change 'info' to 'debug' ###

log4j.rootLogger=debug, stdout

mybatis

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN" "http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>

    <!--通过properties标签加载外部properties文件-->
    <properties resource="jdbc.properties"></properties>

    <!--自定义别名-->
    <typeAliases>
        <typeAlias type="com.itheima.domain.User" alias="user"></typeAlias>
        <typeAlias type="com.itheima.domain.Order" alias="order"></typeAlias>
        <typeAlias type="com.itheima.domain.Role" alias="role"></typeAlias>
    </typeAliases>

    <!--数据源环境-->
    <environments default="developement">
        <environment id="developement">
            <transactionManager type="JDBC"></transactionManager>
            <dataSource type="POOLED">
                <property name="driver" value="${jdbc.driver}"/>
                <property name="url" value="${jdbc.url}"/>
                <property name="username" value="${jdbc.username}"/>
                <property name="password" value="${jdbc.password}"/>
            </dataSource>
        </environment>
    </environments>
    

    <!--加载映射文件-->
    <mappers>
        <mapper resource="com/itheima/mapper/UserMapper.xml"></mapper>
        <mapper resource="com/itheima/mapper/OrderMapper.xml"></mapper>
    </mappers>


</configuration>

OrderMapper.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.itheima.mapper.OrderMapper">

    <resultMap id="orderMap" type="order">
        <!--手动指定字段与实体属性的映射关系
            column: 数据表的字段名称
            property：实体的属性名称
        -->
        <id column="oid" property="id"></id>
        <result column="ordertime" property="ordertime"></result>
        <result column="total" property="total"></result>
        <!--<result column="uid" property="user.id"></result>
        <result column="username" property="user.username"></result>
        <result column="password" property="user.password"></result>
        <result column="birthday" property="user.birthday"></result>-->

        <!--
            property: 当前实体(order)中的属性名称(private User user)
            javaType: 当前实体(order)中的属性的类型(User)
        -->
        <association property="user" javaType="user">
            <id column="uid" property="id"></id>
            <result column="username" property="username"></result>
            <result column="password" property="password"></result>
            <result column="birthday" property="birthday"></result>
        </association>

    </resultMap>

    <select id="findAll" resultMap="orderMap">
         SELECT *,o.id oid FROM orders o,USER u WHERE o.uid=u.id
    </select>

</mapper>

UserMapper.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.itheima.mapper.UserMapper">


    <resultMap id="userMap" type="user">
        <id column="uid" property="id"></id>
        <result column="username" property="username"></result>
        <result column="password" property="password"></result>
        <result column="birthday" property="birthday"></result>
        <!--配置集合信息
            property:集合名称
            ofType：当前集合中的数据类型
        -->
        <collection property="orderList" ofType="order">
            <!--封装order的数据-->
            <id column="oid" property="id"></id>
            <result column="ordertime" property="ordertime"></result>
            <result column="total" property="total"></result>
        </collection>
    </resultMap>

    <select id="findAll" resultMap="userMap">
        SELECT *,o.id oid FROM USER u,orders o WHERE u.id=o.uid
    </select>

    <!--#############################################################################-->

    <resultMap id="userRoleMap" type="user">
        <!--user的信息-->
        <id column="userId" property="id"></id>
        <result column="username" property="username"></result>
        <result column="password" property="password"></result>
        <result column="birthday" property="birthday"></result>
        <!--user内部的roleList信息-->
        <collection property="roleList" ofType="role">
            <id column="roleId" property="id"></id>
            <result column="roleName" property="roleName"></result>
            <result column="roleDesc" property="roleDesc"></result>
        </collection>
    </resultMap>

    <select id="findUserAndRoleAll" resultMap="userRoleMap">
        SELECT * FROM USER u,sys_user_role ur,sys_role r WHERE u.id=ur.userId AND ur.roleId=r.id
    </select>



</mapper>

5.测试

一对一测试

  @Test
    public void test1() throws IOException {
        InputStream resourceAsStream = Resources.getResourceAsStream("sqlMapConfig.xml");
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream);
        SqlSession sqlSession = sqlSessionFactory.openSession();
        OrderMapper mapper = sqlSession.getMapper(OrderMapper.class);
        List<Order> orderList = mapper.findAll();
        for (Order order : orderList) {
            System.out.println(order);
        }
        sqlSession.close();
    }

一对多测试


    @Test
    public void test2() throws IOException {
        InputStream resourceAsStream = Resources.getResourceAsStream("sqlMapConfig.xml");
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream);
        SqlSession sqlSession = sqlSessionFactory.openSession();

        UserMapper mapper = sqlSession.getMapper(UserMapper.class);
        List<User> userList = mapper.findAll();
        for (User user : userList) {
            System.out.println(user);
        }

        sqlSession.close();
    }

多对多测试


    @Test
    public void test3() throws IOException {
        InputStream resourceAsStream = Resources.getResourceAsStream("sqlMapConfig.xml");
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream);
        SqlSession sqlSession = sqlSessionFactory.openSession();

        UserMapper mapper = sqlSession.getMapper(UserMapper.class);
        List<User> userAndRoleAll = mapper.findUserAndRoleAll();
        for (User user : userAndRoleAll) {
            System.out.println(user);
        }